In general, glasses as industrial products are produced by heating raw material powders prepared in a prescribed mixing proportion in a crucible or a tank furnace at temperatures higher than the liquidus temperature to form a homogeneous mixture in the melting state and then quenching the mixture. In the production of glass, the glass is usually made transparent by, for example, a means in which the bubbles in the melt formed from adsorbed gas in the raw materials and the gas generation during the reaction are removed by thoroughly elevating the temperature of the melt to reduce the viscosity of the melt and floating the gases or air bubbles.
However, in the case of producing a glass from silica as the raw material, because of its high melting point, the temperature cannot be elevated to an extent effective for the bubble-removing owing to the restrictions in refractories of the crucible or furnace or other reasons and if the temperature is elevated excessively, gases are generated by the volatilization of the raw material per se and the reaction between the raw material and the crucible form bubbles. Therefore, the above-described method cannot be employed. For the reasons set forth above, a method for the production of a transparent quartz glass using silica as the raw material is restricted to either one of a generally known Verneuil method, a zone melting method, or a vacuum melting method.
The Verneuil method is a method in which a silica powder is gradually fed into an argon-oxygen plasma flame or an oxygen-hydogen flame and melted for glass formation and the resulting melt is deposited onto a stand, and the generated gases are dissipated from the surface.
The zone melting method is a method in which a porous body composed of a silica fine powder is prepared and melted from one end thereof in a band-like state for glass formation, and the generated gases leave through the unmelted porous body.
The vacuum melting method is a method in which a rock crystal powder prepared to have a particle diameter of about 100 .mu.m is placed in a crucible and melted in a vacum heating furnace for glass formation, and the generated gases are removed by force.
However, with respect to the Verneuil method and the zone melting method, it is well known that an extremely long period of time is required for producing one glass block and its productivity is poor, and especially in the case of the Verneuil method, the yield is extermley low, 30% to 40%. Further, when the argon-oxygen plasma flame is employed as a heat source, though a glass having a small number of residual OH groups and a relatively small number of bubbles can be obtained, the energy cost is high, whereas when the oxygen-hydrogen flame which is cheap in the energy cost is employed, a product having a large number of residual OH groups is obtained. Still further, since the shape of ingots which can be produced is restricted to a cylindrical and slender one, there is a further disadvantage to the subsequent processings.
According to the vacuum melting method, though a relatively large-sized ingot having a small number of residual OH groups and a high viscosity at high temperatures can be obtained, since the raw material powder filled in a vessel such as a crucible is melted for glass formation, not only is there a disadvantage to the debubbling but also a reaction gas caused by the contact with the vessel is generated and the resulting glass has a relatively large number of bubbles. Therefore, those having a high quality cannot be obtained. Further, because of the use of the rock crystal powder, there is a disadvantage to the raw material supply due to exhaustion of resources.